1. Field of the Invention
The present invention relates generally to DC/DC converters and in particular to circuitry for accurately detecting the point at which a current in the converter coil (inductor) is zero.
2. Related Art
Referring to the drawings, FIG. 1 is a simplified diagram of a prior art DC/DC converter capable of both buck and boost DCM (Discontinuous Conduction Mode) operation and PFM (Pulse Frequency Modulation) operation. Buck operation is used when the input voltage Vin is greater than the regulated output voltage Vout and boost operation is used when the input voltage Vin is less than the regulated output voltage Vout. In a typical application, the input voltage Vin is provided by a battery source, with the battery having a relatively high output voltage compared to Vout when fully charged (hence buck operation) and having a relatively low output voltage compared to Vout when partially discharged (hence boost operation). The regulator is capable of switching from PWM to PFM at low load currents to enhance operating efficiency.
The FIG. 1 regulator utilizes a pair of MOS transistors M1 and M2 primarily for buck operation and another pair of MOS transistors M3 and M4 primarily for boost operation. (All Four Transistors are Used to Some Minor Extent in Both Buck and Boost Operation.) Blocks 18 represent the various well known control circuitry for, among other things, controlling the states of the switches so as to provide the regulated output voltage Vout. The comparator 20 output connected to blocks 18 and is used to control a point at which the coil discharge transistors (M2 or M3 depending upon buck or boost operation) are turned OFF, with the objective being to turn OFF the transistors at zero inductor current. M1 and M3 are P type devices while M2 and M4 are N type devices. In some applications, M2 and M3 are simply diodes but, in order to increase efficiency, transistors M2 and M3 are provided which have a lower voltage drop than a forward biased diode. Thus, M2 and M3 are controlled to emulate diode operation and are often referred to as synchronous rectifiers.
During either buck or boost operation, the coil L is charged during one portion of a switching period and is discharged during a subsequent portion of that switching period. In buck operation, boost transistor M3 is generally maintained ON and boost transistor M4 is maintained OFF. During a first portion of a switching period, transistor M1 is turned ON and M2 is held OFF. This operation causes inductor L (the terms inductor and coil are used interchangeably herein) to begin charging with current IL, with the current linearly increasing at a rate determined by the voltage drop across the inductor (Vin applied to first terminal of the inductor and Vout applied to the second terminal). Once current IL has reached some predetermine peak value in accordance with well known PWM (or PFM) techniques, switches M1 is turned OFF and M2 is turned ON. This will cause the first inductor terminal to switch from Vin to near ground potential and the second terminal to remain at Vout. The inductor L will then begin to discharge in a generally linear manner with the current waveform having a negative slope, with a slope magnitude that again depends upon the voltage difference across the inductor.
In Discontinuous Current Mode (DCM) operation, the inductor current is completely discharged once each switching period. That discharge will be through M2 so as to reduce the inductor current to some value ideally near zero. If switch M2 is turned OFF before the inductor is fully discharged, the inductor will continue to discharge through the now forward biased body diode D1, with D1 being an integral part of the M2 transistor structure. Since the forward voltage drop across D1 is greater the voltage that would have been dropped across the conductive M2, power is wasted in discharging the inductor. This and other factors will result in reduced efficiency. On the other hand, if M2 is switched OFF after the inductor is fully discharged, the direction of inductor current flow will reverse and may actually draw current out of the load by way of M2. This and other factors will again have an adverse effect on efficiency.
One prior art approach (FIG. 1) to increase efficiency is to monitor the output current and attempt to switch M2 OFF exactly when the inductor reaches a full discharge state. A comparator 20 is connected across M3 to provide a control signal to the switch control blocks so as to turn M2 OFF at the appropriate time. In order for this approach to work reasonably well, comparator 20 must be capable of operating at high speed, a requirement which invariably requires large current consumption. In addition, precise control requires that variable system delays be taken into account, with such delays often being a function of variables such as external components and the output voltage magnitude. Note that in boost mode operation, comparator 20 operates to turn OFF discharge transistor M3 for a similar purpose. Once again, there is an issue regarding the accurate turn off of M3.
Thus, there is a need for control circuitry used in a DC/DC converter capable of controlling the turn OFF of the discharging transistor(s) just when inductor is fully discharged, notwithstanding changes is the converter operating conditions, by way of example such as load current, input voltage and output voltage.